The invention relates generally to methods and apparatus for diagnostic imaging, and more particularly to a trigging apparatus that responds to changing input frequencies to allow a digital phase lock loop to rapidly lock to a desired output frequency.
In certain known diagnostic imaging systems, such as CT imaging systems, an x-ray source transmits x-ray beams towards an object of interest. The x-ray beams pass through the object being imaged, such as a patient or baggage. The beams, after being attenuated by the object, impinge upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. Attenuation measurements from the detectors are acquired separately for each detector element and collectively define a projection data set or transmission profile.
The x-ray source and the detector array are rotated on a gantry within an imaging plane around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, e.g., projection data set, from the detector array at one gantry angle is referred to as a “view.” A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. The projection data sets are processed to construct images that correspond to two-dimensional slices taken through the object at various angles.
Conventional CT medical imaging and baggage scanning systems include a triggering apparatus for triggering the acquisition of projections. The triggering apparatus may use an encoder to provide a predetermined number of pulses per gantry revolution. A phase locked loop (PLL) may be used to multiply and filter the encoder signals to provide a stable output frequency. For instance, existing CT systems may use an analog PLL that includes a digital filter on the input to stabilize input frequency parameters to provide filtering to reduce or maintain the jitter of an output signal within an acceptable range. Jitter is typically defined as an irregular random movement of the output signal above or below a desired frequency level. Analog PLLs with a external digital filter are typically slow to respond to variations in input frequency to provide adequate output filtering. To accurately acquire projections, the rate of rotational speed change of the CT gantry is limited by the response time of the PLL, limiting the ability to rapidly change rotational speeds which is desirable for diagnostic and image reconstruction purposes. Furthermore, typical commercial systems limit the number of output pulses per gantry rotation from the PLL to a defined set of values, e.g., three values. Moreover, communications errors between the PLL input source, for instance, an encoder and the PLL may cause a scan to abort because of a missing or jittered acquisition trigger.
It is desirable to provide a diagnostic imaging system, for example, a CT system that has a trigger interpolation system that provides increased system stability to non-ideal events such as noise, communication errors and impaired electronic functionality. It would be further desirable for the system to be able to specify triggers per each gantry rotation at any arbitrary number while maintaining output jitter within a predetermined acceptable range. It is also desirable to provide a CT system that has the ability to interpolate digital acquisition system (DAS) triggers to allow the CT system to avoid aborting a scan by being able to continue scanning when an intermittent error or instantaneous error occurs. The PLL may be set to a desired frequency and in the event of a missing or late input pulse it would be desirable to interpolate the output pulses to stabilize the PLL and prevent additional unwanted jitter from being introduced into the output signal